This question about cartridge fuses came up in our shop:
This "Tron" (Bussman FNQ) fuse is blown, and we don't have the right value in stock. Can I substitute this "Dual-Element" (Bussman FNM) in its place?

The fuse was protecting a pair of condenser fan motors and a control-voltage transformer primary. I said "Yes." Then, just to confuse myself, I looked the two styles up on the Bussman web site to see how they differ.

From the Bussman www site:

The FNQ (Tron) fuses:
A fuse with a built in delay that allows temporary and harmless inrush currents to pass without opening, but is so designed to open on sustained overloads and short circuits.

The FNM (Dual-Element) fuses:
Fuse with a special design that will utilize two individual elements in series inside the fuse tube. One element, the spring actuated trigger assembly, operates on overloads up to 5-6 times the fuse current rating. The other element, the short circuit section operates on short circuits up to their interrupting rating.

Ummm ... Okay ...
Even after getting my tweezers out and picking the language over, I get the idea that these two fuses do pretty much the same thing. Does anyone know anything about the real physical or functional differences between them? TIA ... !

09-25-2012, 04:58 PM

gravity

id say they do the same thing. is this a fuse for a carrier unit by chance?

09-25-2012, 05:19 PM

fixitman

Quote:

Originally Posted by gravity

... is this a fuse for a carrier unit by chance?

Nope.

09-25-2012, 06:54 PM

hvac69

FNQ is rated for 500 VAC the FNM is only rated at 250 VAC

09-26-2012, 08:02 AM

Russ57

Not only is the voltage rating higher on the FNQ series, the interupting current rating is WAY higher. If you don't know the importance of I.C. rating then best you make sure you always put the factory speced part back in place.

They are not remotely close to being functionaly equivilant. The FNQ fuse also costs more.

09-26-2012, 12:17 PM

fixitman

Quote:

Originally Posted by Russ57

Not only is the voltage rating higher on the FNQ series, the interupting current rating is WAY higher. If you don't know the importance of I.C. rating then best you make sure you always put the factory speced part back in place.

They are not remotely close to being functionaly equivilant. The FNQ fuse also costs more.

Thanks to hvac69 and Russ57 for their replies. Better than the Bussman web site at pointing out the differences between two of their products.

In this particular case, I knew that the voltage involved was 208V / 120V-to-ground. At that voltage, I'm satisfied that the FNM is an adequate replacement. Good to know that it would matter at the higher voltages, though.

09-27-2012, 07:53 AM

timebuilder

Quote:

Originally Posted by fixitman

This question about cartridge fuses came up in our shop:
This "Tron" (Bussman FNQ) fuse is blown, and we don't have the right value in stock. Can I substitute this "Dual-Element" (Bussman FNM) in its place?

The fuse was protecting a pair of condenser fan motors and a control-voltage transformer primary. I said "Yes." Then, just to confuse myself, I looked the two styles up on the Bussman web site to see how they differ.

From the Bussman www site:

The FNQ (Tron) fuses:
A fuse with a built in delay that allows temporary and harmless inrush currents to pass without opening, but is so designed to open on sustained overloads and short circuits.

The FNM (Dual-Element) fuses:
Fuse with a special design that will utilize two individual elements in series inside the fuse tube. One element, the spring actuated trigger assembly, operates on overloads up to 5-6 times the fuse current rating. The other element, the short circuit section operates on short circuits up to their interrupting rating.

Ummm ... Okay ...
Even after getting my tweezers out and picking the language over, I get the idea that these two fuses do pretty much the same thing. Does anyone know anything about the real physical or functional differences between them? TIA ... !

While they both handle temporary higher currents (usually on startup) the dual element fuse handles more significant higher currents, as it is designed specifically for that purpose

A DE fuse is a good replacement choice.

As mentioned, voltage ratings and AIC are important. The proper voltage and a generous AIC rating should be part of your thinking on this.

09-27-2012, 10:59 AM

fixitman

Quote:

Originally Posted by timebuilder

While they both handle temporary higher currents (usually on startup) the dual element fuse handles more significant higher currents, as it is designed specifically for that purpose

A DE fuse is a good replacement choice.

As mentioned, voltage ratings and AIC are important. The proper voltage and a generous AIC rating should be part of your thinking on this.

Thanks for your reply.
Right - motor-starting inrush current was what I was focused on.

I'm completely baffled by the circuit it protects, I have to say ... either one of those condenser fans wiping out could kill control power to the unit. D'oh!